Coated particles, especially microparticles, have many fields of application: advanced composite materials, medicine, paints and other coatings with controlled optical properties. In composites, conformal coatings can improve chemical compatibility or adhesion between the filler particles and the matrix material. Such coatings can also act as barriers, preventing reaction between particles and the surrounding environment. Barrier coatings can also be semi-permeable; allowing slowed or controlled reactions, such as time-release medicine microparticles. Coatings on microparticles can also be tailored to produce specific desired optical properties. By carefully tuning coating properties, particles can be designed to reflect specific colors, for instance. The coated particles can then be mixed into a matrix material to produce “paint” or a composite with very specific optical properties.
Such diverse applications demand a coating system that can produce a wide variety of coatings on a wide variety of particles. Physical vapor deposition (PVD), including techniques such as sputtering and evaporation, can produce such varied coatings using metals and ceramics. PVD also allows for careful control of deposition rate, and thus coating thickness and composition. Multilayer coatings and custom alloy coatings can be created using multiple PVD sources. When coating particles, the coatings must be conformal and uniform on large numbers of particles. Combining a PVD coating system with a fluidized bed system allows large numbers of particles to be conformally and uniformly coated as required.
A fluidized bed of solid particles can be formed by applying a mechanical vibration to the bed. Under the right vibration conditions, the particle bed reaches a dynamic state in which the particles move rapidly and randomly throughout the material volume, and the system appears to flow like a fluid. The frequency, amplitude and waveform of the vibration can be used to tune the behavior of the fluidized bed. Different bed masses, particle sizes, shapes, and densities can be fluidized by tuning the driving frequency and amplitude of the vibration. The random, dynamic mixing achieved in a fluidized bed makes it an ideal platform for creating full-coverage particle coatings from line-of-sight deposition techniques such as sputtering. Additionally, the random motion of the particle bed results in uniform and consistent particle coatings even if the deposition source is spatially non-uniform.
Previous systems have utilized vibratory motion to create a fluidized bed inside a PVD chamber by placing vibratory sources within the PVD chamber, see for example and U.S. Pat. Nos. 5,506,053 and 6,288,837 to Hubbard and U.S. Pat. No. 7,312,097 to Hammerbacher et al. However, as many PVD operations require controlled environments and sub-ambient pressures, these previous methods have required specialized vibratory sources that can withstand vacuum conditions and material released from the deposition source. These constraints limit the size and flexibility of the coating system, and add considerable expense and maintenance requirements.
The present invention provides a novel method and a novel apparatus for transferring vibratory motion from outside to inside the vacuum chamber to produce a fluidized bed. The method allows the use of non-vacuum rated vibratory sources, thus reducing complexity and cost compared to systems in the previously described patents. The present system also allows a wider variety of coatings to be produced on particles having a larger range of shapes and sizes, compared to systems of the previously described patents. Sputter deposition using multiple sputter sources is employed to produce single- and multi-layer coatings on the fluidized particles. Simultaneous co-sputtering can also produce custom alloy coatings on the fluidized particles. The technique also allows for the randomization of a very high number of particles (for example, millions) whereas simpler shaking or tumbling techniques can only effectively agitate tens or hundreds of particles.
Therefore, the inventors have provided an improved apparatus and methods for coating particles and/or fibers within a chamber which vibratory motion for a fluidized bed is generated externally to a chamber and transmitted into the chamber via a mechanical linkage.